This application relates generally to combustors and, more particularly, to gas turbine combustors.
A present thrust of gas turbine engine technology seeks to attain reduced emissions of nitrogen (NOx) and hydrocarbon compounds. Techniques for accomplishing such reduced emissions often result in reduced thermodynamic efficiency or substantially increased capital costs.
NOx compounds are produced by a reaction of the nitrogen in an oxidant at elevated temperatures conventionally found in the combustors of a gas turbine engine. NOx formation can be reduced by reducing the maximum flame temperature in the combustor. Injection of steam into the combustor reduces the maximum flame temperature in the combustor at a cost in thermodynamic efficiency. Penalties must also be paid in water use, including water treatment capital outlay and operating costs. The amount of steam injection, and its associated cost, rises with the amount of NOx reduction desired. Some states and foreign countries have announced targets for NOx reduction that infer such large quantities of steam that this solution appears less desirable for future systems.
Reduction or elimination of hydrocarbon emissions is also attainable by ensuring complete combustion of the fuel in the combustor. Complete combustion requires a lean fuel-oxidant mixture. As the fuel-oxidant mixture is made leaner, a point is reached at which combustion can no longer be supported. Thus, significant research has been conducted to reduce the maximum flame temperature, while still permitting efficient operation of the combustor.
Dry low NOx combustion, which limits NOx formation by lowering flame temperatures through fuel/oxidant optimization, has been developed. Dry low-NOx combustors control fuel and oxidant mixing to create larger and more branched flames, reduce peak flame temperatures, and lower the amount of NOx formed. In principle, there are three stages in a conventional dry low-NOx combustor: combustion, reduction, and burnout. In the initial stage, combustion occurs in a fuel-rich, oxygen-deficient zone where the NOx is formed. A reducing atmosphere follows, where hydrocarbons are formed and react with the already formed NOx. In the third stage, internal oxidant staging completes the combustion.
While dry low NOx combustion in a gas turbine has produced gains in the effort to reduce NOx emissions, dry low NOx combustion is sensitive to changes in oxygen content in the combustor oxidant. Oxidant supplied to the combustor is usually composed of ambient air brought in through a compressor that has varying oxygen content due to dilution with ambient water vapor and possibly additional water vapor from an evaporative cooler or other device that cools inlet air through evaporation of water. In addition, other diluents (such as steam, nitrogen or liquid water) are occasionally added to the oxidant before or during the combustion process. Thus, it is desirable to determine the oxygen content in the total oxidant.